Cultivated sunflower (Helianthus annuus L.) is a diploid species (2n=34) grown in many temperate, semi-dry regions of the world as a source of oil and confectionery seeds. Oil types of sunflowers contain 40 to 48 percent oil in the seed. Sunflower oil is valued as an edible oil, because of its high unsaturated fat level and light color. Sunflower oil is used for salads, cooking oil or for margarine. The protein content of sunflower meal prepared from seeds after oil extraction is useful as livestock feed. The seeds from both oil and confectionery varieties of cultivated sunflower are useful as bird food.
Only a relatively few herbicides have been found and developed for selective weed control in cultivated sunflower. These herbicides include alachlor, S-ethyl dipropylcarbamothioate (EPTC), ethalfluralin, trifluralin, pendirnethalin, chloramben, imazamethabenz-methyl, sethoxydim and sulfentrazone. Additional weed control treatments are needed to provide a better spectrum of weed control and to reduce the development of weed resistance to herbicides.
Among the weeds insufficiently controlled by herbicides presently used in cultivated sunflower are members of the Orobanchaceae family. These weeds are obligate root holoparasites of a number of broadleaf plants, including sunflower. Particular Orobanche species afflicting sunflower include Orobanche aegyptiaca Pers., O. ramosa L., O. minor Sm., O. cumana Wallr. and O. cernua Loefl. O. cumana Wallr. and O. cernua Loefl. (alternative names for the same species) is a severe pest in sunflower in eastern Europe and has been spreading through southern Europe. Orobanche presents. a worldwide risk, and some species such as O. minor have appeared as exotics in the United States. Orobanche species are very difficult to eliminate, because, except for their flower parts, they live in the soil, and their seeds are minute, prolifically produced, easily dispersed and very long-lived. Thus, herbicides presently used in sunflower generally provide inadequate control.
Since the discovery of sulfonylurea herbicides over twenty years ago, over two dozen sulfonylureas have been commercially developed for selective weed control in a wide variety of crops (The Pesticide Manual, Eleventh Edition, C. D. S. Tomlin, ed., British Crop Protection Council, Surrey, U.K., 1997). Sulfonylurea herbicides. have as an essential molecular structural feature a sulfonylurea moiety (—S(O)2NHC(O)NH(R)—). The sulfonyl end of the moiety is connected either directly or by way of an oxygen atom or an optionally substituted amino or methylene group to a cyclic or acyclic group. At the opposite end of the sulfonylurea bridge, the amino group, which may have a substituent such as methyl (R being CH3) instead of hydrogen, is connected to a heterocyclic group, typically a symmetric pyrimidine or triazine ring, having one or two substituents such as methyl, ethyl, methoxy, ethoxy, methylamino, dimethylamino, ethylamino and the halogens. As the mode of action of sulfonylurea herbicides is inhibition of the enzyme acetolactate synthase (ALS) found in plants but not animals, sulfonylurea herbicides provide a valued combination of excellent efficacy against weeds and very low toxicity to animals.
While sulfonylureas have been developed for selective weed control in a variety of crops, ordinary varieties of cultivated sunflower are generally insufficiently tolerant for sulfonylureas to be useful for selective weed control in sunflower crops. However, preemergence application of a low dose (2 to 6 g/ha) of chlorsulfuron has been reported to result in 75-85% control of O. cernua with sunflower tolerance (L. Garcia-Torres et al., Weed Research 1994, 34, 395-402). Although sulfonylurea herbicides have thus been shown to have effect on Orobanche species, the sensitivity of ordinary varieties of sunflowers to sulfonylureas prevents use of higher application rates to give better control of Orobanche. 
Greater application rates of sulfonylurea herbicides could be used to control Orobanche as well as other weed species if varieties of sunflower more resistant to sulfonylureas could be developed. To be easily incorporated in breeding programs combining desirable traits, the trait for sulfonylurea tolerance should be highly heritable (i.e. dominant or semi-dominant). Induced mutagenesis has been used to produce sulfonylurea resistance in soybeans, as discussed in U.S. Pat. No. 5,084,082, but this approach has not been reported for sunflower, which has dissimilar seed morphology compared to soybean. Furthermore because 99% of induced mutations are recessive (W. Gottschalk & G. Wolff Induced Mutations in Plant Breeding, Springer-Verlag, New York, 1983, particularly p. 12), dominant mutations are extremely rare. To find dominant herbicide resistance mutations typically requires screening many thousands of mutagenized seeds.
Accordingly there is a need to be able to selectively control Orobanche and other weeds using sulfonylurea herbicides. Applicants have conducted an extensive research program to find dominant or semi-dominant mutant traits providing sulfonylurea resistance in cultivated sunflower.